The modern information revolution has led to an ever-increasing demand for data storage systems. As a case in point, CD and DVD disks represent successful high-volume data storage technologies. One major advantage of these technologies is that reading or writing of data is accomplished by shining light on the disk so there is no physical contact between the media and the optical head. However, the total storage capacity of these disks is limited by the size of the smallest marks on the surface of the media that can be read by the wavelength of light employed. Many attempts have been made to develop data storage systems with progressively smaller marks. However, the required equipment is prohibitively expensive, and the data access rates tend to be unacceptably slow.
One way to increase the storage capacity of a medium is to record the information depth-wise, rather than just on the surface. There could be used holography, two-photon optics, and similar methods for illuminating media in three dimensions, with the goal of producing marks in three dimensions, and thereby providing very high data capacity systems.
Bleaching and photoreactions (e.g., photochromicity) of organic dyes has also been used as a means to record optical data, both in a single layer in writeable CD-type media, and depth-wise (dissolved in a bulk piece of polymer). However, a large amount of optical power is required in these systems to produce readable marks, therefore the rate of recording of such media is slow. Also, many photochromic systems also tend to fade over time.
Holographic recording has also been achieved by optically induced birefringence in suitable polymers, a process which relies on photo-alignment of the side chains within the polymers. Once again, a large amount of optical power is required, and this process is inefficient and slow. In addition, the fidelity of the recorded information may degrade with time since optically induced orientation tends to relax over time in polymers.
JP 2000-086588 discloses a recording medium using changes in circular dichroism based on the interconversion of chiral norbornadiene and quadricyclane derivatives. However, this technique requires enantiomerically enriched compounds that are difficult to synthesize
U.S. Pat. No. 5,759,721 discloses a holographic recording medium, which uses a photopolymerization technique, which can also be used for recording information optically in three dimensions.
There is a problem with this process, however, in that photopolymerization is usually accompanied by shrinkage of the material, which is a consequence of the process of forming new chemical bonds among the constituents. Dimensional changes that occur on writing limit the resolution that can be achieved, and reduce the data capacity of the medium. In addition, photopolymerization generally requires the use of low molecular weight reactants so that media made from these materials tend to be undesirably soft or sticky. Furthermore, the most common method of photopolymerization, free radical polymerization, is subject to interference by atmospheric oxygen, which causes undesirable inconsistencies in the process.
U.S. Pat. No. 6,569,600 B2 discloses an optical recording material based on a chemical transformation (e.g. isomerization) of a reactant in a polymeric binder, which is initiated by one-electron oxidation of the reactant via electron transfer to a sensitizer. While the electron transfer sensitized optical recording materials of U.S. Pat. No. 6,569,600 B2 offer potential advantages over prior art, the quantum yields (number of molecules transformed per photon absorbed) resulting from the electron transfer process in polymer binders are generally less than 2. Thus, relatively long exposure times and/or relatively high light intensities are required to record information.
Co-pending, commonly-assigned U.S. patent application Ser. No. 10/700,873 filed Nov. 4, 2003, discloses an optical recording article based on electron transfer sensitized isomerization in a polymeric matrix that is formed in-situ.
Photolytic chain isomerizations induced by triplet sensitizers are disclosed in J. Amer. Chem. Soc. 95, 2738 (1973) and in Nouveau J. Chem. 1, 363 (1977). While isomerization quantum yields as high as 10 are calculated by extrapolation, measured quantum yields do not exceed 2. Furthermore, isomerizations are carried out in solution rather than in a polymeric matrix. The very high viscosities and restricted motions encountered in polymeric media can severely reduce efficiencies of many photochemical processes.